Over the last few years, several optical data storage and laser recording media have been disclosed. For example, an article in Optical Engineering, Volume 15, Number 2, Mar.-Apr., 1976, page 99, discusses properties of a large number of such media. One major facet of optical data storage media is that the recorded surface must be protected from fingerprints, dust, or other surface contaminants which can cause errors in reading recorded data. This is particularly true when the data storage medium is reflective in nature.
Optical data storage may be achieved in many ways. Where the recording media is reflective in nature, data may be read by reflection, phase shift, scattering or transmission through the media. Some optical data storage media are absorptive rather than reflective. These may be read by transmission of light through clear areas. In the case of reflective data storage and recording materials, most are comprised of a thin metallic film disposed over some supporting substrate. These reflective metallic films are generally electrically conducting, though some reflective materials are not. An example of an electrically non-conducting reflective optical data storage and recording medium is that disclosed in application Ser. No. 55,270 filed July 6, 1979, by Bouldin and Drexler, now U.S. Pat. No. 4,278,756. The metallic film is then covered or encapsulated with a transparent protective coating or coverplate so that data images are not obscured by fingerprints, dust particles, or other common surface contaminants. After encapsulation any surface contamination or damage affects only the outer surface of the protective layer and not the data on the reflective surface. The diameter of the laser beam used either to read or record is much larger at the outer surface than at the data spot, so that imperfections on the surface of the coverplate have little effect on the read or recording signal.
Dijkstra, in U.S. Pat. No. 4,188,433, issued Feb. 12, 1980, discloses a disc shaped optical data storage carrier which is provided with a reflective metal layer which is protected by a coverplate having a thickness of 0.2 to 1 millimeter. This coverplate is attached to the reflective metal layer by an ultraviolet curable lacquer. The difficulty Dijkstra was attempting to overcome is the inherent surface drying problem involved in coating lacquers. Previously, thick lacquer layers, used to separate dust and scratches from the data surface, displayed considerable internal stress so that the lacquer readily peeled off the reflective surface. In addition, when a polymerization initiator is present in the lacquer layer, it will be more active in the outer parts of the lacquer layer than in the deeper situated parts. In addition, curing time for most lacquers is comparatively long, thus allowing the lacquer to attack the supporting substrate. As a result of these problems, Dijkstra employed unltraviolet curable lacquers which had decreased curing times and were more tenacious.
Hamisch, in U.S. Pat. No. 3,990,084, issued Nov. 2, 1976, discloses an information carrier suitable for recording and storage. This information carrier had a data recording and storage layer composed of a mixture of bismuth, antimony and selenium and a different element of the group selenium and tellurium. Information was recorded on this layer by burning away selected regions with a laser. A coating of lacquer may be applied to this information carrying layer either prior to the recording of information or subsequent thereto. This lacquer layer is applied to protect the information carrying layer against damage and dirt.
One problem heretofore not dealt with by the prior art has been the formation of Newton rings when a coverplate contacts a reflective data storage surface without being bonded to it. Newton rings are concentric rings, not usually perfect circles, produced when one plane surface is held in contact with another, by interference between directly transmitted light and that transmitted after being reflected back and forth between layers imperfectly in contact. In the above cited prior art, Newton rings were avoided by having lacquer-bonded contact with the coverplate and the reflective surface. However, the use of bonding lacquers slows the manufacturing process and introduces design restrictions to permit this bonding procedure. An alternative method of avoiding Newton rings is by spacing the coverplate so that it does not contact the reflective surface but is very close to it. This method may be adequate if the reflective surface is stiff and the small spacing between the coverplate and the reflective surface can be maintained relatively uniformly. If the reflective surface is on a thin flexible film, maintaining this small spacing over a large area presents a problem.
Another problem heretofore not dealt with in the prior art is providing replaceable coverplates over optical data storage media. Thus if the coverplate encapsulating valuable stored optical data is damaged by scratches, melting or distortion it may be replaced, thus preserving the optical data.
An object of the present invention is to find a general method of avoiding Newton rings created by placing a coverplate on a reflective data storage and recording layer. Another object of the invention is to provide a sterile environment for the reflective data storage and recording layer. Yet another object of the invention is to provide an economical and practical optical data storage and recording medium.